test with standalone tests

With some help from Stephen (upstream):
I've just had a look at the debian/rules in
https://sources.debian.org/src/libtommath/1.1.0%7Erc5-1/
and you do run testme.sh but that's not really required as that's doing
more than you want to test!
the easiest way would be to do `make test_standalone` and then execute
`./test` which will run the testsuite for the platform you're compiling
for. (that one's also not invoking `make clean` )